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Difficulty: MediumIPv4 Static Routing

A Cisco router receives an IPv4 packet destined for host 192.168.2.50192.168.2.50. The router's routing table contains a static route configured as `ip route 192.168.2.0 255.255.255.0 10.1.1.2`. Which sequence correctly orders the internal packet processing and recursive routing lookup steps executed by the router from first to last?

  1. 1The router compares destination IP 192.168.2.50192.168.2.50 against routing table entries using longest prefix match logic.
  2. 2The routing table matches static route entry `192.168.2.0/24` and identifies 10.1.1.210.1.1.2 as the next-hop IP address.
  3. 3The router initiates a recursive lookup in the routing table to find a route to next-hop IP 10.1.1.210.1.1.2.
  4. 4The recursive lookup matches directly connected subnet `10.1.1.0/24` and identifies egress interface GigabitEthernet0/1.
  5. 5The router resolves the Layer 2 MAC address for 10.1.1.210.1.1.2 via ARP and forwards the encapsulated frame out GigabitEthernet0/1.

Answer

The correct order of steps for processing a packet requiring a recursive static route lookup is: First, match the destination IP address against the routing table using longest prefix match. Second, extract the static route entry pointing to next-hop IP 10.1.1.2. Third, perform a recursive lookup for 10.1.1.2. Fourth, resolve the egress interface to GigabitEthernet0/1 via the connected route. Fifth, perform ARP resolution for 10.1.1.2 and encapsulate/forward the frame out GigabitEthernet0/1.
The correct sequence begins with comparing the incoming packet's destination IP address against the routing table using longest prefix matching. Upon matching the static route for 192.168.2.0/24, the router retrieves the next-hop IP address (10.1.1.2). Because the static route was configured with only a next-hop IP address rather than an exit interface, the router must execute a second (recursive) lookup to resolve how to reach 10.1.1.2. The recursive lookup matches a directly connected route (10.1.1.0/24 on GigabitEthernet0/1), establishing the outbound interface. Finally, ARP resolves the MAC address corresponding to 10.1.1.2, allowing the router to encapsulate and transmit the frame.

Step-by-Step Solution

1
Perform initial routing table lookup for destination IP 192.168.2.50192.168.2.50.
Matches static route prefix `192.168.2.0/24` based on longest prefix match.
Routers evaluate incoming packet destinations against all active routes in the routing table to find the best match.
2
Extract the next-hop IP address 10.1.1.210.1.1.2 from the matched static route.
Determines that packets must be sent toward next-hop IP address 10.1.1.210.1.1.2.
When a static route points to an IP address rather than an interface, the router must identify where to send the traffic next.
3
Execute a recursive lookup in the routing table for IP 10.1.1.210.1.1.2.
Searches for an active routing entry covering IP 10.1.1.210.1.1.2.
The router needs to determine which exit interface connects to the network containing next-hop address 10.1.1.210.1.1.2.
4
Resolve next-hop IP 10.1.1.210.1.1.2 to connected subnet route `10.1.1.0/24` on GigabitEthernet0/1.
Identifies GigabitEthernet0/1 as the physical outbound interface.
The recursive process ends when the next-hop IP resolves to a directly connected interface.
5
Perform ARP resolution for 10.1.1.210.1.1.2 and transmit frame.
Encapsulates packet in Ethernet frame with next-hop MAC address and transmits out GigabitEthernet0/1.
Layer 3 packet processing concludes with Layer 2 address resolution and frame transmission out the designated egress interface.

Key Concept

IPv4 Static Route Recursive Lookup Process
Estimated Time:1m 30s
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Topics

IPv4 Static Routing

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